1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved means and process for securing an expanded foil mask to a shadow mask support structure that extends from the inner surface of a faceplate. Color tubes of various types having the tension foil mask can be manufactured by the process, including those used in home entertainment television receivers. The process according to the invention is of particular value in the manufacture of medium-resolution, high-resolution, and ultra-high resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the screen and as the beam-passing apertures move out of registration with their associated phosphor elements on the screen. When heated, the tension mask distorts in a manner quite differently from that of the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located closely adjacent to the faceplate. The front assembly primarily comprises the faceplate with its screen which consists of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three electron beams lands only on its assigned phosphor deposits.
2. Prior Art
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube faceplate in conjunction with a thin foil tension shadow mask. A front panel is disclosed that has an inner ledge that forms a continuous path around the screen. No details as to the means for securing a foil mask to the inner ledge are provided, other than a statement that the mask is "sealed" to a ledge.
The use of a laser as a means for welding a foil mask on a shadow mask support attached to the inner surface of a faceplate is described in application Ser. No. 832,493 filed Feb. 21, 1986, now U.S. Pat. No. 4,730,143, of common ownership herewith, and titled "Improved Color Cathode Ray Tube Having a Face-Plate-Mounted Metal Frame with a Welded-on Tension Foil Shadow Mask." No information concerning the welding process is given, other than the statement: "The welding process may be electrical resistance welding or laser welding."
In U.S. Pat. No. 4,591,344 to Palac, of common ownership herewith, a method of making a color cathode ray tube is disclosed in which a frame on which a shadow mask is stretched has indexing means cooperable with registration-affording means on a faceplate. The assembly provides for multiple registered matings of the faceplate and mask during photoscreening operations. The sealing areas of the faceplate and the frame are joined in a final assembly operation such that the frame becomes an integral constituent of the cathode ray tube.
A mask registration and supporting system for a cathode ray tube having a rounded faceplate with a skirt for attachment to a funnel is disclosed by Strauss in U.S. Pat. No. 4,547,696 of common ownership herewith. The skirt of the faceplate provides the necessary Q-distance between the mask and the screen. A frame dimensioned to enclose the screen comprises first and second space-apart surfaces. A tensed foil shadow mask has a peripheral portion bonded to a second surface of the frame. The frame is registered with the faceplate by ball-and-groove indexing means. The shadow mask is sandwiched between the frame and a stabilizing or stiffening member. Following final assembly, the frame is permanently fixed in place within the tube envelope between the sealing lands of the faceplate and a funnel, with the stiffening member projecting from the frame into the funnel.
In referent copending application Ser. No. 831,696, now U.S. Pat. No. 4,721,488, of common ownership herewith, there is disclosed an apparatus for tensing a shadow mask foil. The apparatus comprises a pedestal having registration-affording means, and a tensing structure that includes a fixture comprising a pair of collars for clamping the edge of a shadow mask foil to support and maintain the foil taut. An anvil is provided for engaging a peripheral portion of the clamped foil to induce deflection of the foil and, thereby, a predetermined tension in the foil. Following a photoscreening process, the mask is secured to shadow mask supports extending from the faceplate by, for example, welding by laser.
There have been a number of disclosures of tensed foil masks and means for applying and maintaining mask tension. Typical of these is the disclosure of Law in U.S. Pat. No. 2,625,734, which addresses the construction of a taut, planar, foraminous mask, and the mounting of the mask and target (the screen on the faceplate) as a unitary assembly within the envelope. The thin metal is clamped in a frame, and the mask is heated and placed under screw tension. Upon cooling, the metal contracts and the mask is thus rendered taut and held in tension by the frame. A photographic plate is used in a process for applying phosphor elements to the faceplate screening surface to provide an interchangeable mask system, rather than using a shadow mask mated with the faceplate to serve as an optical stencil during photoscreening. Law in U.S. Pat. No. 2,654,940 discloses means for stretching and captivating masks formed of mesh screens by frame means.
In a journal article, there is described means for mounting a flat tension mask on a frame for use in a color cathode ray tube having a circular faceplate with a curved viewing surface. In one embodiment, the mask, which is also circular, is described as being welded to a circular frame comprised of a 1/8-inch steel section. The frame with captivated mask is mounted in spaced relationship to a phosphor-dot faceplate, and the combination is assembled into the tube as a package located adjacent to the faceplate. ("Improvements in the RCA Three-Beam Shadow Mask Color Kinescope," by Grimes et al. IRE, January 1954; decimal classification R583.6.)